Rocks can be put into three fundamentally different types: Igneous rock Sedimentary rock Metamorphic rock IGNEOUS ROCKS-Igneous rocks are crystalline solids which form directly from the cooling
of magma. This is an exothermic process (it loses heat) and involves a
phase change from the liquid to the solid
state. The earth is made of igneous rock - at least at the surface where
our planet is exposed to the coldness of space. Igneous rocks are given
names based upon two things: composition (what they are made of) and texture
(how big the crystals are).
SEDIMENTARY ROCKS-In most places on the surface, the igneous rocks which make up the majority
of the crust are covered by a thin veneer
of loose sediment, and the rock which is made as layers of this debris
get compacted and cemented together. Sedimentary rocks are called secondary,
because they are often the result of the accumulation of small pieces broken
off of pre-existing rocks.
METAMORPHIC ROCKS-The metamorphics get their name from "meta" (change) and "morph"
(form). Any rock can become a metamorphic rock. All that is required is
for the rock to be moved into an environment in which the minerals which
make up the rock become unstable and out of equilibrium with the new environmental
conditions. In most cases, this involves burial which leads to a rise in
temperature and pressure. The metamorphic changes in the minerals always
move in a direction designed to restore equilibrium. Common metamorphic
rocks include slate, schist, gneiss, and marble.